In traditional analog-to-digital conversion (ADC), a signal is sampled uniformly, with constant time between samples, at a rate that is at least twice its maximum signal frequency. However, at higher sampling rates (e.g., on the order of 1 GHz and above) ADC performance is relatively poor. As a result, traditional analog-to-digital conversion is limited to signals at lower frequencies. For signals at higher frequencies, an alternative is to instead sub-sample the signal, at a rate that is less than twice the maximum signal frequency. This causes the sub-sampled signal to have a frequency spectrum that is “folded” with respect to the original. As a result, signals at frequencies that are separated by multiples of the sampling frequency will fold onto one another.
Furthermore, as signal frequency increases, the temporal uncertainty of each sample (i.e., timing jitter) becomes a limiting factor in ADC performance. This has motivated the use of pulsed lasers as optical samplers in recent years, due to their ability to provide timing jitter that is orders of magnitude better than electronic samplers. However, sub-sampling using pulsed lasers has the same problem that signal frequencies fold onto one another.
A potential solution to the frequency ambiguity introduced by sub-sampling is to use non-uniform sub-sampling, in which the sample-to-sample timing is non-uniform. This provides additional information that can be used to determine the signal frequency from the sub-sampled signal and has been demonstrated using both electronic sampling and optical sampling. However, non-uniform, electronic samplers are limited by timing jitter. In conventional systems, the limited optical bandwidth of an electronically-generated optical pulse source can limit non-uniform optical sampling. Further, conventional systems do not distinguish all signal frequencies in the sub-sampled frequency spectrum.
Features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.